Nanostructured materials are generally regarded as materials having very small grain feature size, typically in the range of approximately 1-100 nanometers (10−9 meters). Metals, ceramics, polymeric and composite materials may be processed in a variety of ways to form nanosized features. These materials have the potential for wide ranging applications, including for example, industrial, biomedical and electronic applications. As a result, a great deal of study is ongoing to gain a better understanding of the characteristics of these materials.
Conventional extrusion formed products are limited to approximately twelve layers. Micro-layer extrusion processes can extend these limitations. Micro-layer extrusion processes that provide methods for obtaining small grain features are described in U.S. Pat. No. 7,690,908, (hereinafter the “'908 patent”) and U.S. Patent Publication 2012/0189789 (hereinafter the “'789 Publication”) both of which are commonly owned by the assignee of the instant application, the disclosures of which are incorporated herein by reference in their entirety. Further examples of extrusion technology are described in U.S. Pat. Nos. 6,669,458, 6,533,565 and 6,945,764, also commonly owned by the assignee of the instant application.
The typical micro-layer product is formed in a sheet. If a tubular product is desired, the microlayer is first formed into a sheet and then made into the tube. This creates a weld line or separation between the microlayers. The '908 patent describes a cyclical extrusion of materials by dividing, overlapping and laminating layers of flowing material, multiplying the flow and further dividing, overlapping and laminating the material flow to generate small grain features and improve properties of the formed product. Examples of the improved properties include, but are not limited to burst strength, tensile strength, tear resistance, barrier and optical properties. The '789 Publication describes extruding a flow of extrusion material in a non-rotating extrusion assembly, forming a first set of multiple laminated flow streams from the extruded flow, amplifying a number of the laminations by repeatedly compressing, dividing and overlapping the multiple laminated flow streams, rejoining the parallel amplified laminated flows, forming a first combined laminate output with nano-sized features from the rejoining; and forming a tubular shaped micro-layer product from the combined laminate output. Such products do not contain a so-called weld line, rather the weld is staggered and integrated into interdigitating layers of the laminate.
Recent reports state that extruded polymer may also form crystal lamellae when the polymeric layer is reduced to a micro or nano dimension. Such crystal lamellae have been observed to influence porosity. See for example Haopeng Wang, Jong K. Keum, Anne Hiltner, and Eric Baer; “Confined Crystallization of PEO in Nanolayered Films Impacting Structure and Oxygen Permeability;” Macromolecules, (2009) 42, 18, 7055-7066. Decreasing layer thickness to the nanometer range, however, does not always increase barrier properties. Gene Medlock and Michail Dolgovskij, (Kuraray America Inc. Pasadena, Tex.), Barrier Performance Of Nanolayer Evoh Film, Tappi Place Conference Seattle, Wash. (May 9, 2012) reported that EVOH nanolayers actually increased oxygen permeability but also increased tear strength. Polymeric processes for producing multi-layer microporous membranes with increased layer blend regions combined with extrudate stretching has produced items with variable pore size and permeation properties such as in United States Patent Publication 2011/0206973, published Aug. 25, 2011, entitled “Multi-Layer Microporous Membranes And Methods For Making And Using Such Membranes.” See also Yu, T. Ph.D. Dissertation, Virginia Tech, 1996 and Johnson, M., Ph.D. Dissertation, Virginia Tech, 2000.